ast week, loud popping sounds triggered a frightening chain of events at J.F.K. Airport. Mass panic spread through two terminals as travelers ran toward exits, sought cover under tables, and broke through secure doors to escape what they believed was a terrorist attack. Given conflicting guidance by security officers and no official directives, many people followed the cues of the mob and ran out onto the tarmac. Eventually, an announcement over the public address system ordered everyone to abandon their bags and evacuate with their hands above their heads. The source of the popping sound, which some attributed to gunfire, was never determined.

Six weeks earlier, on June 26, I was walking toward a security checkpoint at Charles de Gaulle Airport in Paris, preparing to board a flight home. After two announcements requesting that a passenger return to the check-in counter for an unattended bag, an armed guard stepped in front of me, blew a whistle and shouted, “Run that way” as he pointed behind me. Oddly, my first thought was about missing my flight. After a few seconds, I felt afraid and walked swiftly toward the exit with the rest of the crowd. Several minutes later, we heard a loud boom. We were then informed over the loud speaker that the emergency was over. One of the officers told us that the bomb squad had exploded a suspicious, unattended bag. My plane took off on time with me on it.

Why was there panic at J.F.K. and calm in Charles de Gaulle? One difference between the incidents was the immediate response of local authorities. Lapses in communication among law enforcement agencies in New York appeared to fuel the panic, and travelers were left to their own instincts on how to respond. By contrast, in Paris we received rapid, informed direction from police who showed a coordinated response.

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The emotional temperature should have been lower in New York than in Paris. It’s been 15 years since the attacks on the World Trade Center, while France has been lashed by more than a dozen brutal terrorist strikes since the Charlie Hebdo attack in Paris in January 2015. You might think that the increasing frequency of terrorism in France would make people jumpier and more hysterical when faced with an immediate threat. But the opposite was true. Repeated exposure to any stressful situation will desensitize people and cause them to be less emotionally reactive, which might explain the relative calm during the Paris airport event.

Incidents of mass panic or hysteria depend on the mood of the crowd at the time an episode occurs. Over the past 30 years, I have studied many outbreaks of mass panic and hysteria. In all of them, the common denominator is a backdrop of anxiety and stress. Fear and anxiety can spread from person to person like a contagious disease. When people are predisposed to overreact to any fearful stimulus, mass hysteria can instantaneously take over a crowd.

Social contagion can occur anywhere that groups of people gather, and they aren’t limited to the fear of terrorist attacks. I have investigated incidents where large groups of people became ill because they were convinced that an environmental threat was causing them real physical symptoms — headache, pain, numbness, fainting — even when no actual threat existed. On May 20, 1981, elementary school students in Templeton, Mass., were afflicted with nausea, abdominal pain, and shortness of breath during two school assemblies. Following extensive searches, investigators concluded that the illness resulted from a psychological rather than a physical cause.

Sometimes such illnesses can persist for days. Most of the time, though, once the afflicted crowd disperses, the symptoms disappear, probably because they are only “contagious” when new victims observe others falling ill. Rumors about the causes of these outbreaks are common and spread quickly through neighboring communities via social media.

The J.F.K. episode did not involve social contagion of psychosomatic symptoms, but rather the spread of panic, fear, and false beliefs. These have contributed to mass hysteria outbreaks recorded as far back as the Middle Ages, when nuns in isolated convents would meow together at specific times of day for no reason.

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On October 30, 1938, many listeners believed that Martians were invading the United States as they listened to Orson Welles do his “The War of the Worlds” radio dramatization. In 1944, residents of Mattoon, Ill., believed that a “phantom gasser” was spraying poisonous mist into the bedroom windows of teenage girls, causing nausea, vomiting, and burning sensations in their mouths and throats.

In the early 1950s, when people in the state of Washington were on edge about nuclear testing, many believed that cosmic rays or shifts in the Earth’s magnetic field were causing previously unnoticed windshield pits or dings in their cars. Some even blamed it on “supernatural gremlins.” These examples show how a worried group can misinterpret physical phenomena and environmental cues that might otherwise go unnoticed.

In this new era of terrorism, we have a lot to worry about. My guess is that we can expect to observe more of these episodes. Thanks to almost daily reports of terrorist attacks, we’re told to be ever more vigilant of anything suspicious or out of the ordinary. Such attentiveness has saved many lives since 9/11. But at the same time we need more effective ways of stopping the spread of rumors and false beliefs of imminent threats.

When we face uncertainty, our minds crave explanations. Because of the regular and graphic accounts of terrorist explosions and gunfire that can occur almost anywhere — airports, malls, city streets, or concerts — a loud popping sound is no longer interpreted as a burst balloon or an engine backfire but as a signal to flee from impending danger.

In today’s frenzied, digitally connected world, anxiety and panic can spread farther and faster than ever before. We need countermeasures to calm rising fears. Many blame the media for fueling fear and panic, but an important job of the press is to inform the public of events as they emerge. Although sensationalized headlines can sometimes fuel social contagion of panic, the media also have the ability to quell mass fear.

Because we are living in a new era of terrorism, we need new action plans that will help authorities pool resources so they can inform us of real danger and also manage benign events so they aren’t misinterpreted as threats and cause unnecessary mass panic. Policy makers, public health officials, and national and local authorities must become more effective in using social media to inform those at risk about real threats and sensible responses. Just as many know the Heimlich maneuver and CPR to address individual medical emergencies, the public needs a menu of coordinated plans of action to help avert future outbreaks of mass panic.

Truth and knowledge trump anxiety and fear any day.

Gary W. Small, MD, is a professor of psychiatry and biobehavioral sciences and aging at the David Geffen School of Medicine at UCLA. He has written about his research on social contagion and mass hysteria in The New England Journal of Medicine and in his book (coauthored with Gigi Vorgan), “The Other Side of the Couch” (Harper Collins, New York, 2010).